Witnessing Light-Driven Entanglement using Time-Resolved Resonant Inelastic X-ray Scattering*

Quantum computers use entanglement to encode information which requires precise characterization and control of entanglement within their constituent materials. However, quantifying entanglement in many-body materials is a challenge in both theory and experiment. While entanglement witnesses can be extracted from spectroscopic data under equilibrium conditions, this approach cannot extend out of equilibrium and is therefore incompatible with laser control of materials. To address this limitation, we developed a systematic approach for characterizing time-dependent entanglement in nonequilibrium quantum materials using time-resolved resonant inelastic x-ray scattering (trRIXS). This is done by using quantum Fisher information (QFI) as a witness for the lower bound of entanglement depth, thus bypassing the requirement for a full tomography of the wavefunction. To showcase the efficiency of our approach, we applied it to a quarter-filled extended Hubbard model (EHM) and predicted light-enhanced quantum entanglement that we attribute to the proximity to a phase boundary. This advancement establishes the foundation for experimentally witnessing and manipulating entanglement in light-driven quantum materials via experimentally accessible ultrafast spectroscopic measurements.